1. Field of the Invention
The present invention concerns a method for calculation of an orthogonal x-ray attenuation of a subject using a reference x-ray attenuation measured in the preparation for a diagnostic image acquisition by means of computed tomography, as well as an associated device.
2. Description of the Prior Art
A computed tomography apparatus typically has an x-ray acquisition unit with an x-ray radiator and an x-ray detector arranged opposite each other. The x-ray acquisition unit is mounted such that it can rotate around a rotation axis such that, during rotation, x-ray projection images (radiograms) of a subject arranged approximately centrally to the rotation axis can be acquired from different projection directions. An image (tomogram) of one or more slices of the subject is then generated from these x-ray projection images using numerical back-projection methods. In medical computed tomography, the subject to be examined is a body region of a patient.
To support the subject (in particular the patient) in the beam path of the x-ray acquisition unit, a computed tomography apparatus has subject table that can normally be adjusted in terms of height.
The body region of a patient to be examined by means of a computed tomography apparatus generally causes an x-ray attenuation (subsequently designated as attenuation for short), the strength of which is different for different projection directions. A modern computed tomography (CT) apparatus is frequently equipped with an automatic dose control that adapts the radiation intensity to the magnitude of the attenuation dependent on the projection direction, such that an optimally good image quality is achieved with a low x-ray dose. For parameterization of the automatic dose control, the attenuation is conventionally measured in two reference projection directions orthogonal to one another (in particular anterior-posterior (ap) and lateral), for example using two overview radiograms acquired before the actual diagnostic tomographic data acquisition. The measured values of the ap-attenuation and the lateral attenuation are supplied as reference values to the automatic dose control, which adapts the radiation intensity to these reference values during the diagnostic CT acquisition in a manner dependent on the projection direction. The ap-attenuation and the lateral attenuation are obtained in the form of an axial attenuation profile (i.e. attenuation as a function of the axial location along the body) and supplied to the automatic dose control in order to take into account differences of the attenuation in the longitudinal body direction.
In order to accelerate the workflow associated with a medical CT acquisition, a reference measurement of the attenuation is implemented only in one projection direction (in particular in the ap-projection direction) while the orthogonal attenuation (in particular thus the lateral attenuation perpendicular to this) is estimated using the shadow or contour outline of the subject on the x-ray detector. Such a method typically leads to a correct estimation of the orthogonal attenuation (and therewith to a precise adaptation of the x-ray dose) only when the subject is positioned exactly centrally with regard to the rotation axis in the projection direction. An exact positioning of the subject relative to the rotation axis, however, is not always possible, in particular not simultaneously over the entire axial body length. Given an imprecise positioning of the subject, the orthogonal attenuation is systematically underestimated or overestimated in conventional methods, with the result that x-ray projection images with insufficient signal-to-noise ratio are generated, or that an unnecessarily high x-ray dose is applied during the CT acquisition.